User Problem

User would like to stop assignees from changing the card due date once it's been set

Suggested Solutions

Add a manual entry field for start/due dates in automation so a card start/due date can be set based using automation variables.

Examples of automation variables that could be used with a manual entry field:

{cardduedate=wd}
{date+10d}
{cardstartdate+1w}
{customfieldvalue}
{{%Milestone 1 Date}} + {{%Milestone 2 Date}}

Also a suggestion to add additional automation variable to reference the previous card due date if the due date has since changed.
e.g.

{cardduedatebefore}

or

{cardpreviousduedate}

Screenshots of proposed suggestion attached.

Current Workarounds

Watch the board, list or card to be notified of any changes made: https://support.atlassian.com/trello/docs/watching-cards-lists-and-boards

Create an automation rule to alert you when a due date is moved:
"when a due date is moved in a card, send an email notification to ... with subject ... and message ..."